For the forming to give a component, the respective flat steel product to be formed is inserted into a forming machine and then is formed by the machine to give the respective component. This forming may be implemented as cold forming, in other words as forming at temperatures below the recrystallization temperature of the respective steel of the flat steel products, or as hot forming, in other words as forming at operating temperatures which lie above the recrystallization temperature.
One typical example of a forming operation of this kind is deep drawing, in which the flat steel product to be formed is pressed by means of a punch into a die. The shape of die and punch here determine the form which the flat steel product receives as a result of the forming operation.
In any forming operation, there are relative movements between the product to be formed and the forming tool used for shaping in each case. At the same time there is contact between the surfaces of the product and the corresponding surfaces of the forming tool. The tribological system which develops between the tool and the product to be formed is determined by the physical properties of the product to be formed and of the tool, and also by the media present between the product to be formed and the tool. As a result of the relative movement between the forming tool and the product to be formed that makes contact with the forming tool, friction is produced.
In the forming of flat steel products, in particular, this friction may greatly differ locally, because, in the course of forming, the material of the flat steel product is deformed differently in sections and therefore the material of the flat steel product also flows to different extents locally during the deformation. Therefore, especially in the production of components of complex shape by deep drawing or comparable cold forming operations, where generally high degrees of forming are obtained and complex shapes are modeled, there are dynamically changing frictional conditions in which static friction and sliding friction may occur alternately.
The frictional forces which come about in the case of cold forming, in particular, may be high enough to possibly disrupt the continuous running of the shaping operation and to cause incorrect molding of the particular component being formed. At the same time, the unavoidable friction results in considerable tool wear.
Proving particularly critical in this respect are flat steel products to which a zinc-based or aluminum-based protective coating, affording protection from corrosion or other environmental influences, has been applied to the actual flat steel product.
In order to diminish the adverse effects triggered by friction during forming, the surfaces that come into contact with one another during the forming operation are in practice lined with lubricants. Through the use of suitable coating materials it is possible to protect the forming tools and so to extend the tool lives substantially. For this purpose, the lubricant may be applied both to the flat steel product to be formed and to those surfaces of the tool that come into contact with the flat steel product.
Customarily used as lubricants for cold forming are lubricants based on mineral oil, to which various additives may be added in order to optimize their lubricity effect, such as sulfur-containing, phosphorus-containing or chlorine-containing adjuvants. A detailed elucidation of tribology within forming technology is found in section 2.8 of volume 4 “Umformen” [Forming] of the “Fertigungsverfahren” [Fabrication processes] compendium by Prof. Dr.-Ing. Fritz Klocke, Prof. em. Dr.-Ing. Dr. h.c. mult. Wilfried König, 5th edn., 2006, Springer-Verlag Berlin Heidelberg.
Examples of coating materials for cold forming that are based on mineral oil or similar hydrocarbons are described in DE 101 15 696 A1. These coating materials include lubricants with a paraffinic or naphthenic basis, or ester oils with a plant or animal basis.
DE 10 2008 016 348 A1, furthermore, describes a low-friction coating which is intended for application to the particular flat steel product to be formed and which is based on graphite in mineral oil. At high processing temperatures, this low-friction coating is said to ensure effective sliding of the metal between the processing tools.
DE 100 07 625 A1 discloses coating materials based on carbonic esters. The coating materials comprise one or more components selected from the group of the monoesters and/or diesters of mono- or oligophosphoric acids, triglycerides, and fatty acid methyl esters. These components are intended to serve in particular as a substitute for mineral oil hydrocarbons or other petroleum distillates.
DE 699 06 555 T1, lastly, describes a method for applying a layer of zinc hydroxysulfate to galvanized steel sheet. The layer is applied in the form of a solution to the flat steel product, the pH of the solution being greater than or equal to 12 but less than 13. The solution is applied to the galvanized surface of the flat steel product by anodic polarization of the surface. The layer thus produced consists of zinc hydroxysulfate, also called “basic zinc sulfate”.